In recent years, the development of white LEDs having features such as compactness, prolonged service life, low-voltage drive, and freedom from mercury have been forwarded as an alternative to mercury gas excitation-type fluorescent lamps (FLs) and cold cathode ray tubes (CCFLs) which have hitherto been used, for example, general lighting and backlights for liquid crystal display devices.
White LEDs are classified into type 1 in which three color-light-emitting diodes, i.e., a red light emitting diode, a green light emitting diode, and a blue light emitting diode are used in combination to emit white light, and type 2 in which a light emitting diode with a long wavelength ultraviolet (300 to 430 nm) or blue wavelength (460 to 480 nm) light emitting diode as an excitation source is used in combination with a phosphor layer containing a plurality of types of visible light emitting phosphors to emit white light.
In type 1, since three color light sources are used, in order to produce white light, a mechanism for color mixing, for example, the use of a diffusive plate and the provision of a space for diffusion, is necessary rendering the thickness of the backlight large. Further, since three types of LEDs are used, for example, respective separate control circuits are necessary. Accordingly, the number of necessary components is increased, disadvantageously leading to increased cost. On the other hand, in type 2, all of LEDs used may be identical and emit the same color light. Accordingly, the mechanism for color mixing is unnecessary. Further, the use of only one control circuit suffices for contemplated results, and, thus, the number of necessary components can be reduced. Therefore, white LEDs can be produced at low cost.
As described above, white LEDs of type 2 are divided into those in which long wavelength ultraviolet (or purple) light emitting diodes are used, and those in which light emitting diodes, which emit blue light (460 to 480 nm), are used.
When an ultraviolet (or purple) light emitting diode is used, white light is produced by using visible light emitting phosphors of three colors, i.e., red, green, and blue in the phosphor layer. On the other hand, in the case of a blue light emitting diode, in many cases, white light is produced by using a yellow visible light emitting phosphor. It has been becoming apparent that, as compared with the use of the blue light emitting diode, the use of the ultraviolet light emitting diode can realize better color reproduction, because, in the case of the ultraviolet light emitting diode, three color phosphors are used for white light, in the case of the blue light emitting diode, two color phosphors are used for white light.
In white LED of type 2, unlike the excitation of a mercury gas at 254 nm in the prior art, the excitation is carried out at long wavelength ultraviolet (300 to 430 nm) or blue wavelength (460 to 480 nm), and, thus, phosphors used in current FLs and CCFLs cannot in many cases be used.
In particular, LaPO4:Ce,Tb commonly used as a green phosphor in FLs and CCFLs does not substantially emit light at 320 to 430 nm. To overcome this problem, Japanese Patent Laid-Open No. 73052/2000 (patent document 1) uses BaMgAl10O17:Eu,Mn (a composition satisfying a general formula in paragraph [0029] in patent document 2) as a green component. Since, however, the emission wavelength of this material is 515 nm, which is shorter than the emission wavelength 543 nm of the conventional material LaPO4:Ce,Tb, this cannot be fully alternative to the conventional material. Therefore, for example, a problem of lowered color rendering properties takes place in illumination applications.
On the other hand, in liquid crystal display devices, the color reproduction range as the display device is determined by a combination of backlight with a color filter. For example, in a liquid crystal display device provided with backlight using CCFL, mere replacement of CCFL with white LED does not improve the characteristics of the liquid crystal display device. This is because the color filter is designed according to color reproduction range of light emitted from CCFL. In other words, mere change of the light source from CCFL to white LED does not always improve the characteristics of the liquid crystal display device.
For example, U.S. Pat. No. 6,252,254 (B1) (patent document 2) discloses a white LED using YBO3:Ce3+,Tb3+ phosphor (a cerium- and terbium-activated yttrium borate phosphor) as a green phosphor and a Y2O2S:Eu3+, Bi3+ phosphor (europium- and bismuth-activated yttrium oxysulfide phosphor) as a red phosphor. In patent document 2, however, since a blue light emitting diode is used, the color reproduction range particularly in the blue part is disadvantageously fixed by luminescence characteristics of the blue light emitting diode. Accordingly, the color reproduction range is different from that in CCFL, and, thus, when the white LED disclosed in patent document 2 is used in a liquid crystal display device in which the conventional CCFL has been used, the design of the color filter should be changed.
Accordingly, a white LED, which has good color reproducibility, that is, can realize a wide range of regulation of the color reproduction range, and, at the same time, despite the use of a light emitting diode, which emits light by a long wavelength ultraviolet (300 to 430 nm), has a color reproduction range comparable with that in conventional FL or CCFL has been desired.
[Patent document 1] Japanese Patent Laid-Open No. 73052/2000
[Patent document 2] U.S. Pat. No. 6,252,254 (B1)